This invention relates to methods of forming images in which the image elements (e.g., lines) are highly precise and ultraminute. More particularly, the invention concerns a method of forming fine patterns by which fine patterns such as those of fine circuits used in semiconductor elements can be formed with high reliability and moreover on a quantity production scale.
Among the numerous applications with which the present invention is concerned are the printing of electrical wirings and the formation of electrical circuits on substrates or base plates of materials such as glass and ceramics. Heretofore, such forming of image patterns have been carried out by processes such as photolithography, screen printing, and offset printing.
With the advancements in electronics, however, there has been an ever-growing demand for further improvements in the fineness and precision of image elements (lines) of patterns constituting electronic elements and components. Accordingly, methods involving the use of photolithography, by which very fine image elements can be formed, have heretofore been widely adopted. However, the throughputs of such methods relying on photolithography have been poor, whereby the production cost has been unavoidably high.
As a measure for lowering this cost, there has been a need for the development of a printing method which is adaptable to and highly effective for quantity production. The printing methods being utilized at present are the screen printing method, the offset printing method, and the like. The image elements that can be formed by these methods, however, are relatively wide, and it is extremely difficult to obtain image elements of fine widths. In fact, forming such fine elements has been impossible.
For example, the screen printing method comprises forming an ink shielding mask on a mesh-form screen so that the non-masked parts delineate the desired pattern and causing the ink to pass though the non-masked parts thereby to print the desired pattern on an object being printed (hereinafter referred to as "printing object"). By this method, an ink thickness of a number of .mu.m to 20 .mu.m can be obtained. For this reason, printing of resist patterns with excellent erosion resistance becomes possible. However, the practical image element (line) width is of the order of 0.1 to 0.2 mm. Therefore, intricate and fine patterns required for semiconductor integrated circuits and other uses cannot be printed by this method.
The offset printing method mentioned above comprises forming lipophilic parts and hydrophilic parts on a presensitized plate (PS plate), causing the hydrophilic parts to hold water and repel an oil-based ink thereby causing the ink to adhere selectively to only the lipophilic parts to form the desired ink pattern, and transferring this ink pattern onto the printing object. In order to improve the printing adaptability of this method, the general method is to first transfer the ink pattern on the PS plate once onto a rubber blanket and then to retransfer the pattern onto the printing object. By this offset printing, relatively fine image elements can be obtained, but, mainly because the ink transfer is carried out twice, the ink film thickness is thin, being of the order of 1 to 2 .mu.m. As a consequence, there arises the problem of pinholes and line breakages readily occurring in the printed image elements.
Accordingly, numerous proposals and attempts have been made by applying various improvements to the offset printing method to increase the ink film thickness and thereby obtain patterns of excellent erosion resistance. However, increasing the ink film thickness inevitably gives rise to an increase in the width of the printed image elements. As a result, the lower limit of the width of the image elements which can be attained by these printing methods in the present state of the art is said to be of the order of 0.1 to 0.2 mm.
It can therefore be said that, by the printing methods as practiced heretofore in the above described manner, highly precise and fine patterns cannot be formed. One of the major reasons for this is that a printing ink passes consistency and even fluidity in some cases.
The consistency or tackiness of a printing ink is ordinarily an indispensable property for printing but becomes a disadvantage in the case where a precise and fine printed pattern is to be formed. More specifically, in the case where the ink is soft, it becomes stretched under pressure or flows when it is being transferred onto a printing object. Consequently, the figures of ink on the printing plate cannot be maintained, and the image transferred onto the printing object differs considerably from the image initially formed on the printing plate. Moreover, the magnitude of this discrepancy fluctuates greatly with the ambient conditions, whereby it is extremely unstable. For such reasons, conventional printing methods cannot be utilized for patterning which requires fineness and, moreover, high precision.
Furthermore, together with the fluctuation of the width of printing image elements at the time of transfer, the fluctuation of the ink film thickness is also large. Consequently, the reliability of the printing with respect to requirements such as chemical resistance and absence of pinholes in the resulting pattern is also poor. These difficulties have been a great problem.